Conventionally, a screw of a product has been manually loosened and removed, in a case where a product with a part to be screwed is disassembled. In recent years, a so-called easy disassembling method has been researched by a combination of a washer made of a shape memory alloy and a screw at the time of fastening. This technology has been developed, and the method has been come into notice from the point of recycling parts and a resource recovery.
Such an easy disassembling technology has been described or disclosed, for example, in Japanese Patent Unexamined Laid-Open Publication No. 16,713 of 2005. At first, a composite product, which is assembled to insert a washer made of a shape memory alloy into a screw, is prepared. The washer made of a shape memory alloy is designed to memorize so as to be a larger inner diameter than an outer diameter of screw head under a condition of being at a predetermined temperature or more. Then, it is regulated and machined to be a smaller inner diameter to a degree that it can be inserted into an axis of screw at the time of fastening parts.
Thus, after using fully a product which is manufactured by fastening parts with the composite product, the product will be disassembled by heating the washer up at the predetermined temperature or more causing that an inner diameter of a washer is restored to be a large diameter as memorized. Then, the washer made of a shape memory alloy is sprung out of the screw head to disassemble the product within a short period.
On the other hand, large fastening forces (torque) are applied to a screw in order to securely fasten a part for secure at the time of screwing. Then, when it is used by a combination of a washer and a screw and the screw head is machined to be a form of countersunk screw, a washer is easily pulled out of the screw head at the time of disassembling a product. In this way, a screw, of which the head has a form of countersunk screw, is convenient at the time of removing a washer.